Steganography is the practice of concealing data. That is, while most branches of cryptography are focused on encrypting and decrypting messages that might be observed and analyzed by any number of individuals (i.e., not just the sender and recipient), steganography is focused on concealing even the existence of such messages.
In 16th-century France, for example, Girolamo Cardano devised a method for hiding messages within written correspondence by providing the recipient a separate sheet of paper including carefully positioned cut-outs or apertures. When properly aligned with a page of the correspondence, individual letters could be viewed through the apertures, thereby revealing the hidden message. This scheme is generally referred to as the “Cardan grille” method, owing to the grill-like appearance of the perforated sheet of paper.
In the digital era, however, steganography typically involves changing the contents of a file in a way that embeds the secret message (i.e., the “target data”) but is sufficiently subtle to avoid detection. For example, the least-significant bits of digital images or sound files may be manipulated, or imperceptible delays may be added to packets sent over a network. Steganography techniques such as these are unsatisfactory in a number of respects, however, because the source dataset (i.e., the original image or file) must be altered in some way to embed the target data. In some applications, the source data cannot be modified, or modification would provide a clue that the target data was encoded therein. Accordingly, there is a need for improved systems and methods for computer-implemented steganography.